Incomplete Seam Marking
Inspect your model for duplicate vertices and zero area polygons and vertices which are near each other yet do not share a face.  Duplicate vertices are sometimes created by extrusion operations .  You may move  vertices to see if  duplicates are in the same location.  Alt-Clicks should select vertex sequences which can be moved and thus inspected.  Look at the model in wireframe mode and check the vertex count which is displayed at the top in blender.

If you have little time investment in the model consider starting from scratch.  Sometimes for simple models this is faster.

Has a UV Unwrap failed?